fix(extensions): add missing useState import in backend extension React example

Fixes #25192

useState was used on line 286 but was missing from the import statement,
causing an error if users follow the tutorial literally.

Author: mohithshuka

content/manuals/extensions/extensions-sdk/build/backend-extension-tutorial.md

@@ -275,7 +275,7 @@ Replace the `ui/src/App.tsx` file with the following code:
 ```tsx
 
 // ui/src/App.tsx
-import React, { useEffect } from 'react';
+import React, { useEffect, useState } from 'react';
 import { createDockerDesktopClient } from "@docker/extension-api-client";
 
 //obtain docker desktop extension client